The present invention relates to a system which will paint components of an automobile more efficiently and more economically. More particularly, the present invention relates to improvements in a painting system which has an automatic painting device.
Generally, a painting system for components of an automobile includes a cleaning process for eliminating oil from a surface to be painted, a primer process and a finish process for painting a finishing coat on the component of the automobile being painted.
Normally, one of three painting methods are applicable to the painting system. These include a hand painting method in which the component is painted by a worker, an automatic painting method in which the component is painted by an automatic painting device and a combination painting method in which the component is painted by the automatic painting device and remaining portions of the component are painted by a worker.
Generally the automatic painting method or the combination painting method of a conventional painting system includes a plurality of automatic painting devices and a plurality of work stands which can move along the straight transfer line. The components of the automobiles (e.g. bumper covers which are made of steel or plastic) are painted by the automatic painting devices.
However, when the component is mounted on the work stand and the component is painted by having the work stand move along the transfer line, the component frequently has mottles, thin painted portions and saggings on the painted surface. These mottles, thin painted portions and saggings are caused by vibration (i.e., shaking) of the work stand. To prevent these vibrations, the work stand is usually stopped at each painting station and positioned at the correct painting position. As a result, the conventional painting system requires too much time to paint the components or many painting devices to paint the components.
In another conventional painting system which can paint over 20,000 components per month and which has more than three painting devices for painting a component of an automobile, a plurality of painting devices are aligned on the same side of a straight transfer lin or located at both sides of a straight transfer line. A considerable interval on the transfer line is defined between the painting devices. This is necessary to prevent an arm of one painting device from colliding with an arm of a second painting device, or to prevent components from being painted by different color paints sprayed from different painting devices. This type of painting system therefore requires too much space in a shop.
In these painting systems, the required time period for producing one painted component of an automobile is calculated by the following formula: EQU A=X/Y+Z (1)
wherein A is the required time period for completely producing one painted component of the automobile, X is the entire time period required for painting one component of the automobile, Y is the number of the painting devices and Z is the time period required for transferring one component from an initial end of a transfer line of the conventional painting system to a final end of the transfer line.
As the number of the painting devices is increased, the time period required for producing one of the painted components is reduced.
For example, if the entire time period `X` required for painting one of the component is 120 seconds, the time period `Z` required for transferring the compnoent is 20 seconds and the number `Y` of painting devices is 4, this painting system can produce the painted component within 50 seconds. However, if the transfer line is lengthened in proportion to the number of painting devices, then the plant for this painting system is expensive.
To shorten the time period required to produce one of the painted components, five solutions were proposed in private as follows:
The first solution proposed in private was to shorten the time period "Z" required is transfer the component from the initial end of the transfer line to the final end of the transfer line, i.e., to increase the transfer speed of the work stand on which the component is mounted.
The second solution proposed in private was to shorten the entire time period "X" required to paint one of the components of the automobile by improving the efficiency of the spray coating process.
The third solution proposed in private was to increase the speed of movement of the arm of the painting device. This increased the total amount of paint sprayed from the painting device.
The fourth solution proposed in private was to increase the area to be sprayed at one time by the painting device. This also increased the amount of paint sprayed from the painting device.
The fifth solution proposed in private was to increase the number of the painting devices.
However, all of these proposed solutions were inefficient for the following reasons:
In the first proposed solution, the component frequently shifted on the work stand because of the rapid acceleration and deceleration of the word stand. This resulted in the component often being in the wrong position on the work stand. Extra labor was then required to reposition the component on the work stand.
The second proposed solution could not obtain more than 50% efficiency in spray coating according to the painting techniques presently employed.
In the third proposed solution, the normal moving speed of the end of the arm of the painting device depends on both the mechanical speed of the painting device and the electrical control speed of the painting device. A movement of the end of the arm of the painting device of more than 1,000 mm/sec. caused exhaustion of the painting device. This also resulted in inaccurate movement of the end of the arm of the painting device, because the mechanical speed of the painting device could not catch up with the fast electrical control speed of the painting device.
In the fourth proposed solution, an increase in the spray area caused saggings on the painted surface because of variable environmental conditions (e.g. room temperature, viscosity of paint, temperature of the component). Further, it needs extremely expensive facilities to control all of environmental conditions to eliminate this drawback. Furthermore, an uneven surface of the component could not be coated uniformly by spray painting a wide portion of the surface of the component at one time.
The fifth proposed solution is uneconomical because the cost of the plant for the painting system is excessive.